Assimilation and extension of particle image velocimetry data of turbulent Rayleigh-Bénard convection using direct numerical simulations

A novel method for assimilating and extending measured turbulent Rayleigh-Bénard convection data is presented, which relies on the fractional step method also used to solve the incompressible Navier-Stokes equation in direct numerical simula-tions. Our approach is used to make measured tomographic particle image velocimetry (tomo PIV) fields divergence-free and to extract temperature fields. Comparing the time average of the extracted temperature fields with the temporally averaged temperature field, measured using particle image thermometry in a subdomain of the flow geometry, shows that extracted fields correlate well with measured fields with a correlation coefficient of CTT=0.84 . Additionally, extracted temperature fields as well as divergence-free velocity fields serve as initial fields for subsequent direct numerical simulations with and without feedback which generate small-scale turbulence initially absent in the experimental data. Although the tomo PIV data set was spatially under-resolved and did not include any information on the boundary layers, the here-proposed method successfully generates velocity and temperature fields featuring small-scale turbulence and thermal as well as kinetic boundary layers, without disturbing the large-scale circulation contained in the original experimental data significantly. The latter is underpinned by high vertical and horizontal velocity correlation coefficients--computed from velocity fields averaged in time and horizontal x-direction obtained from the measurement and from the simulation without feedback--of Cvv=0.92and Cww=0.91 representing the large-scale structure. For simulations with feedback, the generated velocity fields resemble the experimental data increasingly well for higher feedback gain values, whereas the temperature fluctuation intensity deviates noticeably from the values obtained from a direct numerical simulation without feedback for gain values \alpha \ge 1 . Thus, a feedback gain of \alpha=0.1 was found optimal with correlation coefficients of Cvv=0.96 and Cww=0.95 as well as a realistic temperature fluctuation intensity profile. The xt-averaged temperature fields obtained from the direct numerical simulations with and without feedback correlate somewhat less with the extracted temperature field (CTT~0.6), which is presumably caused by spatially under-resolved and temporally oscillating initial tomo PIV fields reflected by the extracted temperature field

Study on the Influence of Turbulence on Thermal Comfort for Draft Air

Today you can find various standards to assess thermal comfort. However, it lacks on standards for the assessment of thermal comfort in car cabins, particularly in case of high momentum air-flow or draft. Thermal comfort is often perceived differently in cars compared to e.g. in buildings or coaches. With the objective to overcome this problem, an experimental study of the impact of the draft on thermal comfort is carried out. In this paper, we present results of an investigation focusing on the influence of turbulence intensity on thermal comfort. Draft is simulated in a generic car cabin in the following parameter ranges: temperature T=17 ∘C–29 ∘C, velocity U=0.25 m/s–2.5 m/s and turbulence intensity ILT=16% and IHT=32%. The thermal comfort is determined by means of a thermal manikin and infrared thermography to evaluate the thresholds of thermal comfort based on the German Industrial Norms

Artificial saliva aerosol source and detection system for spreading analysis in indoor environments

In the context of the Corona pandemic the investigation of aerosol spreading is utmost important as the virus is transported by the aerosol particles exhaled by an infected person. Thus, a new aerosol generation and detection system is set up and validated. The system consists of an aerosol source generating a particle size distribution mimicking typical human exhalation with particles sizes between 0.3-2.5 µm and an array of Sensirion SPS30 particulate matter sensors. An accuracy assessment of the SPS30 sensors is conducted using a TSI OPS3330, a high-precision optical particle sizer. Low deviations of ±5 % of the particle concentration measured with the SPS30 with respect to the OPS are reported for concentrations below 2’500/cm³ and +10% for particle densities up to 25’000/cm³. As an application example the system is employed in a short distance single-aisle research aircraft Dornier 728 (Do728) located at DLR Göttingen, to investigate the large-scale aerosol-spreading. With this measurement system a determined spreading distance from an index passenger extending one seat row to the front and two seat rows to the back is determined

Experimentelle Untersuchung zum thermischen Komfort im Grenzbereich des Luftzugempfindens in einer generischen Fahrzeugkabine

Ob auf dem Weg zur Arbeit oder in den Urlaub – in den heutigen Industriestaaten verbringt der Mensch einen nicht unwesentlichen Teil seiner Zeit im Fahrzeug. Nicht zuletzt deswegen und aufgrund des gesteigerten Wettbewerbs ist der thermische Komfort im Fahrzeug ein mitentscheidendes Argument für die Profilierung im Wettbewerb der Fahrzeughersteller. Für die Auslegung einer Klimatisierung im PKW ist es das Ziel thermische Bedingungen herzustellen, die von möglichst allen Passagieren als behaglich wahrgenommen werden. In diesem Zusammenhang stellt sich die Frage, wie man den thermischen Komfort in einem Fahrzeug vergleichbar bewerten kann. Hierzu gibt es eine Vielzahl von Methoden und Normen zur Charakterisierung und Bewertung des thermischen Komforts in PKW. Jedoch zeigen Untersuchungen, dass die Normen den thermischen Komfort im Grenzbereich des Zugluftempfindens nur unvollständig beschreiben. Das Ziel dieses Forschungsvorhabens ist daher die Untersuchung der Auswirkung von Zugluft auf den thermischen Komfort in Abhängigkeit der folgenden Einflussgrößen: Lufttemperatur, Strahlung, Luftmassenstrom, Luftgeschwindigkeit und Turbulenzgrad. Hierzu wurden die objektiven Komfortparameter in einer generischen Fahrzeugkabine mithilfe eines thermischen Menschmodells bestimmt und basierend auf der Methode der Äquivalenttemperatur nach EN ISO14505-2 bewertet. Die Ergebnisse der Untersuchung sollen mit den aktuellen Normen verglichen werden. Im Zuge dessen soll außerdem geprüft werden, inwieweit eine Anpassung bestehender Normen unter Berücksichtigung des Luftzugempfindens möglich und sinnvoll ist. Auf der Fachtagung werden wir die Ergebnisse einer Teilstudie zur Untersuchung des thermischen Komforts im Grenzbereich des Zugluftempfindens als Funktion der Anströmgeschwindigkeit und des Turbulenzgrads präsentieren

A probabilistic particle tracking framework for guided and Brownian motion systems with high particle densities

This paper presents a new framework for particle tracking based on a Gaussian Mixture Model (GMM). It is an extension of the state-of-the-art iterative reconstruction of individual particles by a continuous modeling of the particle trajectories considering the position and velocity as coupled quantities. The proposed approach includes an initialization and a processing step. In the first step, the velocities at the initial points are determined after iterative reconstruction of individual particles of the first four images to be able to generate the tracks between these initial points. From there on, the tracks are extended in the processing step by searching for and including new points obtained from consecutive images based on continuous modeling of the particle trajectories with a Gaussian Mixture Model. The presented tracking procedure allows to extend existing trajectories interactively with low computing effort and to store them in a compact representation using little memory space. To demonstrate the performance and the functionality of this new particle tracking approach, it is successfully applied to a synthetic turbulent pipe flow, to the problem of observing particles corresponding to a Brownian motion (e.g., motion of cells), as well as to problems where the motion is guided by boundary forces, e.g., in the case of particle tracking velocimetry of turbulent Rayleigh-Bénard convection

Numerical and experimental study of aerosol dispersion in the Do728 aircraft cabin

The dispersion of aerosols originating from one source, the ‘index’ passenger, within the cabin of the aircraft Do728 is studied experimentally using an aerosol-exhaling thermal manikin and in Reynolds-averaged Navier–Stokes simulations (RANS). The overall aim of the present study is the experimental determination of the aerosol spreading for the state-of-the-art mixing ventilation (MV) and to evaluate the potential of alternative ventilation concepts for controlling the aerosol spreading in RANS. For MV, the experiments showed that the ratio of inhaled to exhaled aerosol particles drops below 0.06% (volume ratio) for distances larger than two seat rows from the source. However, within a single row, the observed ratio is higher. Further, the dispersion is much weaker for a standing than for a seated index passenger. High air exchange rates and a wellguided flow prevent a dispersion of the aerosols in high concentrations over larger distances. Additionally, the positive effect of a mask and an increased air flow rate, and especially their combination are shown. In the complementary conducted RANS, the advantages of floor-based cabin displacement ventilation (CDV) which is alternative ventilation concept to MV, regarding spreading lengths and the dwell time of the aerosols in the cabin were determined. The obtained results also underline the importance of the flow field for the aerosol dispersion. Further, additional unsteady RANS (URANS) simulations of the short-term process of the initial aerosol cloud formation highlighted that the momentum decay of the breathing and the evaporation processes take place within a few seconds only

Measurement of the turbulent heat fluxes in mixed convection using combined stereoscopic PIV and PIT

The results of simultaneous measurements of velocity and temperature fields in a turbulent mixed convection airflow are analyzed and discussed. To access local temperature and velocity fields in airflows, we present a combination of stereoscopic particle image velocimetry and particle image thermometry. The obtained flow fields make it possible to determine the local convective heat fluxes, thus givinginsight into the dynamics of plumes and Taylor-Görtler-like vortices. The evaluated mean local heat fluxes further reveal that the main convection roll transports a substantial amount of heat alongthe coolingplate and back to the heated bottomplate. Yet, the associated mean turbulent heat fluxes remain positive as they are dominated by the correlation of the temperature and the vertical velocity component. More specifically, a statistical analysis of the local heat fluxdistribution reveals that Taylor-Görtler-like vortices lead to more skewed distributions of the turbulent convective heat fluxes than plumes

Untersuchung zum thermischen Komfort im Pkw für den Grenzbereich des Luftzugempfindens

In den letzten Jahren ist ein deutlicher Anstieg in Bezug auf wissenschaftliche Untersuchungen zum Themenkomplex „komfortable und energieeffiziente Fahrzeugklimatisierung“ zu beobachten. Die Ursache für das gesteigerte Interesse ist der vermehrte Einsatz von Elektromobilität und die Weiterentwicklung des autonomen Fahrens. Für elektrisch betriebene PKWs ist eine effiziente Klimatisierung besonders wichtig, da diese die Reichweite des Fahrzeugs direkt beeinflusst. Auch beim autonomen Fahren ergeben sich in Zukunft ganz neue Anforderungen an die Fahrzeugklimatisierung und deren Integration. Aufgrund der zu erwartenden, völlig neu strukturierten Fahrzeuginnenräume werden Belüftungskonzepte benötigt, die den neuen Anforderungen an Design und Komfortempfinden gerecht werden

Energy Consumption Evaluation of a Novel Individual Heating System using Infrared Panels for Long Distance and Regional Trains

There is an increasing urgency to shift road transportation of passengers to more environmentally friendly modals such as railways. Individual acclimatization, a commonplace for cars and airplanes is still fiction for railways and one challenge for the modal shift. A joint collaboration between Deutsche Bahn Systemtechnik (DB-ST) and the German Aerospace Center (DLR) conducted a series of experiments at the "Demonstrator for Innovation for Traveller Comfort and Air Conditioning" (DIRK) in Minden to analyse the capability of individual heating elements, especially infrared panels, to maintain or increase the thermal passengers’ comfort, as well as the possibility to reduce the cabin temperature in case of heating, thus allowing a reduction of the heating energy demand while at least maintaining the same thermal comfort level. For this purpose, a system architecture was developed which consists of a control and a measuring system. The control system allows to set the comfort levels of various heating elements by each user via an individual tablet. The measuring system records the electrical power of the respective seats and allows them to be evaluated. This paper introduces the system architecture designed for controlling and measuring the electric energy demand of the heating elements, as well as the power characterization of infrared panels related to the input signal